Mitral valve replacement (MVR) has been performed on 48 patients as primary or secondary treatment of severely symptomatic patients with resting and/or provokable pressure gradients across the left ventricular outflow tract (LVOT) secondary to idiopathic hypertrophic subaortic stenosis (IHSS). Indications for MVR include: 1) septal thickness less than 18 mm; 2) persistent LVOT obstruction after a prior adequate left ventriculomyotomy and myectomy (LVM&M); 3) atypical septal morphology; and 4) severe mitral regurgitation secondary to ruptured chordae tendinae or papillary muscle. Intraoperative echocardiography has provided definition of septal morphology allowing selection for MVR. There have been 3 (6.2%) perioperative deaths, one a result of hepatic failure one suspected to be caused by prosthetic valve malfunction, and 2 secondary to infection. Three patients (6.9%) died after hospital discharge, two suddenly and one of congestive heart and respiratory failure. One patient had a late central embolus. Symptomatic improvement to NYHA functional class I or II has occurred in 86% of 36 patients returning for postoperative evaluation. Excellent relief of both resting and provokable gradients has been demonstrated. Four patients continue to be symptomatic (FC III) and have been shown to have abnormal coronary vascular resistances with no reserve indicating the presence of severe small vessel disease. Thus relief of LVOT obstruction does not always relieve symptoms of chest pain and fatigue. Long-term follow-up will be necessary to assess late mortality and morbidity which will be compared to the well-known results of LVM&M used for palliation in IHSS for the past 26 years.